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a
This comparison is only done if the first (neighboring) AS is the same in the two paths; the MEDs are
compared only if the first AS in the AS_SEQUENCE is the same for both paths.
b
If you entered the bgp always-compare-med command, MEDs are compared for all paths.
c
Paths with no MED are treated as "worst" and assigned a MED of 4294967295.
7
Prefer external (EBGP) to internal (IBGP) paths or confederation EBGP paths.
8
Prefer the path with the lowest IGP metric to the BGP if next-hop is selected when synchronization is
disabled and only an internal path remains.
9
The system deems the paths as equal and does not perform steps 9 through 11, if the following criteria is
met:
a
the IBGP multipath or EBGP multipath are configured (the maximum-path command).
b
the paths being compared were received from the same AS with the same number of ASs in the AS
Path but with different NextHops.
c
the paths were received from IBGP or EBGP neighbor respectively.
10
If the bgp bestpath router-id ignore command is enabled and:
a
if the Router-ID is the same for multiple paths (because the routes were received from the same
route) skip this step.
b
if the Router-ID is NOT the same for multiple paths, prefer the path that was first received as the
Best Path. The path selection algorithm returns without performing any of the checks detailed here.
11
Prefer the external path originated from the BGP router with the lowest router ID. If both paths are
external, prefer the oldest path (first received path). For paths containing a route reflector (RR) attribute,
the originator ID is substituted for the router ID.
12
If two paths have the same router ID, prefer the path with the lowest cluster ID length. Paths without a
cluster ID length are set to a 0 cluster ID length.
13
Prefer the path originated from the neighbor with the lowest address. (The neighbor address is used in
the BGP neighbor configuration and corresponds to the remote peer used in the TCP connection with
the local router.)
After a number of best paths is determined, this selection criteria is applied to group's best to determine the
ultimate best path.
In non-deterministic mode (the bgp non-deterministic-med command is applied), paths are compared
in the order in which they arrive. This method can lead to the system choosing different best paths from a set
of paths, depending on the order in which they were received from the neighbors because MED may or may
not get compared between the adjacent paths. In deterministic mode, the system compares MED between
the adjacent paths within an AS group because all paths in the AS group are from the same AS.
Weight
The weight attribute is local to the router and is not advertised to neighboring routers.
If the router learns about more than one route to the same destination, the route with the highest weight is
preferred. The route with the highest weight is installed in the IP routing table.
Border Gateway Protocol IPv4 (BGPv4)
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Table of Contents
Troubleshooting
